A commercially available cobalt-base alloy, sometimes referred to as X-40 alloy, has been used for several years in the manufacture of components for gas turbine engines. For example, it has found a relatively wide application in the manufacture of conventionally cast turbine vanes. One specification for X-40 alloy defines its composition, by weight, as 0.45 - 0.55% C, 24.5 - 26.5% Cr, 9.5 - 11.5% Ni, 7 - 8% W, up to 2% Fe, up to 1% each of Si and Mn, up to 0.25% Zr, up to 0.04% S, with the balance Co. Thus, such alloy composition is characterized by relatively high carbon content and the absence of aluminum which was generally believed in this alloy to contribute to decreasing such properties as nil-strength temperature and stress rupture. Although the element Al was not intentionally added to such alloy, an analysis of production heats has shown the presence of up to about 0.4% Al as an impurity.
Many thousands of gas turbine engine articles have been manufactured from X-40 alloy by the conventional casting process in which generally uniformly dimensioned, sometimes called "equi-axed", grain structure is produced and the time of contact between molten alloy and the mold or cores with which it comes in contact is relatively short. However, with the development of the directional solidification process, for example of the type described in U.S. Pat. No. 3,897,815 -- Smashey, or the type described in U.S. Pat. No. 3,494,709 -- Piearcey, the disclosures of which are incorporated herein by reference, casting materials and conditions were dramatically changed. For example, such casting process requires relatively long times of contact between molten alloy and ceramic cores, which includes silica to provide the core with sufficient properties to withstand the directional solidification process. Because of the high carbon content of X-40 alloy, there was experienced reaction of the molten metal with silica in the core to form a gaseous oxide of carbon and liquid-phase Si. This reaction resulted in deterioration of the casting surface about the area of reactivity.